We have identified the major histocompatibility complex of the rabbit, the RL-A complex, but we cannot produce the monospecific antisera necessary to type animals from outside our colony, because of the limited number of independent haplotypes found within our animal population. This inability to provide typing antisera has restricted the usefulness of the rabbit as a transplantation model. We propose to produce these monospecific typing antisera, to identify the various loci within the RL-A complex, and to develop the non-inbred rabbit as a useful model for transplantation studies. The role of the major histocompatibility complex in the control of the immune response, cell communication, and susceptibility to certain diseases has again raised questions of the significance of polymorphism and selective advantage at this complex genetic region. We have found evidence suggesting that certain related haplotypes of the RL-A complex have a selective advantage within the genetic and environmental conditions of our colony. We wish to verify this selective advantage in order to provide possible insights into the relationship between the major histocompatibility complex and disease.